"As America prepares to pick our president for the next four years — and as Florida prepares once again to play a decisive role — I'm confident that President Barack Obama is the right leader for our state and the nation," he wrote. "I applaud and share his vision of a future built by a strong and confident middle class in an economy that gives us the opportunity to reap prosperity through hard work and personal responsibility. It is a vision of the future proven right by our history."

The executive director of Sen. Jim DeMint's Senate Conservatives Fund wrote a scathing letter to the group's supporters about Crist's endorsement of the president.

"Charlie Crist's endorsement of Barack Obama illustrates a larger point about moderate Republicans — they can't be trusted," Matt Hoskins wrote. "Moderate Republicans don't believe in our principles. Instead, they use the Republican Party to achieve their political objectives."

Crist left the GOP in 2010, when it became clear he was headed for a bad loss to now-Sen. Marco Rubio in the Republican Senate primary. Running as an Independent, he proceeded to lose a three-way race that also featured then-Rep. Kendrick Meek (D).

"If the Party no longer serves their purposes, they will turn on it faster than you can say Arlen Specter," Hoskins added. "The Washington establishment blames conservatives for hurting Republicans at the ballot box, but we're not the ones who leave the Party when we lose elections."